An image sensor is a semiconductor device used to convert optical images to electric signals. Image sensors may be classified as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS).
A CCD image sensor stores electric charge carriers and transferred from metal oxide silicon (MOS) capacitors.
A CMOS image sensor may be provided with a plurality of unit pixels whereby each unit pixel includes one light sensing device such as a photodiode and a plurality of MOS transistors. The CMOS image sensor may be formed using CMOS technology and may include a control circuit and a signal processing circuit as peripheral circuits. Moreover, red, green and blue optical signals may be sequentially detected by the MOS transistors, and outputted using a switching method.
The CMOS image sensor may offer several advantages such as low power consumption, low manufacturing cost, and high degree of integration.
The CMOS image sensor may include an optical detection unit for detecting light and a logic circuit unit for processing the detected light into an electric signal as data. Attempts have been carried out to increase the ratio (i.e., fill factor) in area between the optical detection unit and the image sensor, which in turn, may increase the optical sensitivity of the device. It may not be fundamentally possible, however, to remove the logic circuit unit, and thus, such attempts are limited in the restricted area.
In order to increase the optical sensitivity, a method of forming and using micro lenses has been proposed which uses a light collector for changing the route of light incident on other regions excluding the optical detection unit, such that light is collected in the optical detection unit. Additionally, an image sensor for detecting a color image may be fabricated such that color filters can be sequentially arranged in an array on the optical detection unit. The color filter array (CFA) may include three color filters from the group of red, green, blue, yellow, magenta and cyan. In addition, micro lenses may be formed on the color filter array to increase the optical sensitivity of the image sensor.
As illustrated in example FIG. 1, a unit pixel of a 4-T CMOS image sensor may include a photodiode (PD) for detecting an optical signal and four n-channel metal oxide semiconductor (NMOS) transistors. The four transistors may include transfer transistor Tx, reset transistor Rx, drive transistor Dx and select transistor Sx. Transfer transistor Tx transmits optical charges generated by the photodiode (PD) to a floating sensing node. Reset transistor Rx discharges the optical charges stored in the floating sensing node for signal detection. Drive transistor Dx acts as a source follower while select transistor Sx performs switching and addressing functions. VDD is drive source voltage and DC gate is a load transistor that may apply uniform voltage to a gate potential of the transistor such that uniform current flows.